We have been investigating the properties of a group of proteins called Tau antigens that bind to the SV40 T Ag in virus-infected and transformed cells. Work in several laboratories has shown that these proteins can be isolated from a number of different cell lines, including cells transformed by retroviruses, papovaviruses, herpesviruses, adenoviruses or chemicals, mouse embryo and embryonal carcinoma cells, and, in small quantities, from normal uninfected cells. We have determined the relationship among Tau antigens isolated from various lines of SV40-transformed cells, SV40-infected monkey cells, and cells of embryonic origin by comparing their methionine-labeled tryptic peptides by 2-dimensional mapping. All of the evidence accumulated so far suggests that Tau antigens are species-specific, evolutionarily conserved proteins. Tau antigens isolated from uninfected, untransformed cells (i.e., embryo cells or pirmary and continuous cell lines) resemble tne analogous protein made in transformed cells of the same animal species. Analysis of SV40-transformed cells that have reverted to a normal (flat) morphology showed that the presence or absence of Tau antigens in these cells correlated with the presence or absence of the SV40 T Ag. This indicated that T Ag was involved in the induction or stabilization of these cellular proteins.